In the oil and gas industry, companies employ many data-gathering techniques. The most detailed, albeit localized, data comes from well logging. During the well-drilling process, or shortly thereafter, drillers pass logging instruments through the well bore to collect information about the surrounding formations. The information is traditionally collected in “log” form, i.e., a table, chart, or graph of measured data values as a function of instrument position.
Acoustic logging tools provide measurements of acoustic wave propagation speeds through the formation. There are multiple wave propagation modes that can be measured including compressional and flexural. Taken together, the propagation speeds of these various modes often indicate formation density and porosity, properties that are useful for determining the location of hydrocarbon gases and fluids.
Acoustic logging measurements are also valuable for determining the velocity or slowness of structure of subsurface formations, which is useful for obtaining accurate images of the subsurface formation structure. Subsurface formations are often anisotropic, meaning that the acoustic waves' propagation speed depends on the direction in which the waves propagate. Many formations exhibit acoustic anisotropy with the rotational axis of symmetry orthogonal to the borehole. This anisotropy is often called azimuthal anisotropy because the material properties are different at different azimuths around the borehole. (This type of anisotropy is also known as horizontal transverse isotropy (“HTI”) because the axis of symmetry is horizontal in a vertical well.) Azimuthal anisotropy is useful for characterizing stress fields, optimizing plans for developing and draining a reservoir, determining fracture orientations and extent, and detecting near well bore damage.
However, it is difficult to accurately represent such anisotropy while conveying relevant data for timely evaluation. Without adequate representation, reservoirs cannot be delineated from surrounding formations and the presence of formation boundaries, laminations, and fractures, cannot be detected. As such, reservoir engineers cannot formulate a production strategy that maximizes economic value in a timely manner.
It should be understood, however, that the specific embodiments given in the drawings and detailed description thereto do not limit the disclosure. On the contrary, they provide the foundation for one of ordinary skill to discern the alternative forms, equivalents, and modifications that are encompassed together with one or more of the given embodiments in the scope of the appended claims.